1. Field of the Invention
The present invention relates to a method for assembling a rocket propulsion motor, and more particularly to a method for assembling a rocket propulsion motor, capable of achieving a sealing between the assembled portions of the rocket propulsion motor.
2. Description of the Prior Art
Rocket propulsion motors are well known as devices for supplying energy required for carrying flying objects or guided missile systems to designated positions or target points. A typical construction of such rocket propulsion motors is illustrated in FIGS. 1 and 2.
FIGS. 1 and 2 shows a general rocket propulsion motor using a solid propellant. As shown in the drawings, the rocket propulsion motor comprises a motor case 1 having a generally cylindrical shape and containing a propellant 3 therein, a nozzle assembly 2 mounted to a rear end of the motor case 1 and an ignitor 4 mounted to a front end of the motor case 1 and directed to the interior of the motor case 1.
In such a conventional construction, a combustion gas generated upon burning the propellant 3 is exhausted outwardly through the nozzle assembly 2. Since the interior of motor case 1 is at high temperature and pressure during the exhaust of combustion gas, a seal should be assured at assembled portions between the motor case 1 and the nozzle assembly 2. If such a seal is not assured, the assembled portions may be exposed to fumes of high temperature and pressure or the seal function is failed, thereby causing the overall rocket system to be adversely affected. In some cases, there is a dangerous explosion of the rocket itself.
Now, a conventional method for assembling such a rocket propulsion motor will be described, in conjunction with FIG. 3 which is an enlarged view of the assembled portions between the motor case and the nozzle assembly.
As shown in FIG. 3, the motor case 1 has at its inner periphery surface a liner 5, an insulating member 6 and a heat resistant member 7. The motor case 1 also has at its rear end a after dome flange 8 at which a plurality of threaded holes 8a uniformly spaced are formed. On the other hand, the nozzle assembly 2 has at its inner periphery surface a heat resistant member 7' and at its outer periphery surface a first O-ring 9 and a second O-ring 10. The nozzle assembly 2 also has at its other periphery surface a nozzle flange 12 at which a plurality of throughout holes 12a uniformly spaced are formed. In assembling, each throughout hole 12a is aligned with the corresponding threaded hold 8a so that a bolt 11 having a high tensile strength is threadedly received in the threaded hole 8a via the throughout hole 12a, as will be described hereinafter.
In assembling the motor case 1 and the nozzle assembly 2 with the above-mentioned constructions, first, surfaces of the heat resistant members 7 and 7' which will come into contact with each other are cleaned. Thereafter, a silicon sealant layer 13 is coated to a predetermined thickness over the cleaned surfaces of heat resistant members 7 and 7'. The silicon sealant layer 13 may comprise a silicon sealant commercially available under the trademark designation "RTA-88" mixed with a setting agent such as dibutyl tin dilaurate (DBT).
The mixing of silicon sealant and setting agent is carried out at atmosphere. The coating of silicon sealant layer 13 over the contact surfaces of heat resistant members 7 and 7' is achieved using a spatula.
Thereafter, the motor case 1 and the nozzle assembly 2 are positioned such that each throughout hole 12a of the nozzle assembly 2 is aligned with the corresponding threaded hole 8a of the motor case 1. Accordingly, a plurality of bolts 11 can be threadedly received in the corresponding threaded holes 8a via the corresponding throughout holes 12a. By fastening the bolts 11, the assembling of the motor case 1 with the nozzle assembly 2 is completed.
Since the assembling of the motor case 1 with the nozzle assembly 2 is carried out at atmosphere in accordance with this conventional method, air may be presented in a space defined between the first O-ring 9 and the silicon sealant layer 13 at the contact surfaces of the assembled portions. As the bolts 11 are fastened, the air is compressed and this compressed air can not escape outwardly through the first O-ring 9, but escapes through the silicon sealant layer 13, thereby forming an air passage 15 as shown in FIG. 4A or air pockets 16 as shown in FIG. 4B. Such air passage 15 and air pockets 16 still remain after the setting of silicon sealant layer 13.
In FIGS. 4A and 4B, the reference numeral 14 denotes an adhesive.
The air passage 15 and air pockets 16 formed in the silicon sealant layer 13 allow fume gases to penetrate the contact surfaces of the assembled portions therethrough during the operation of the rocket propulsion motor, thereby causing the O-rings to be damaged. In severe cases, peripheral elements disposed around the O-rings may be subjected to a fatal damage.
In particular, the process for assembling the rocket propulsion motor is a blind process. Moreover, the air passage and air pockets formed in the silicon sealant layer can be hardly found by a nondestructive test (NDT) carried out after the assembling. Even though such air passage and air pockets have been found, they can not be locally repaired.
These problems about sealing of the assembled portions encountered in assembling the rocket propulsion motor are important things directly affecting the performance of the rocket propulsion motor. For example, the space shuttle explosion accident in January, 1986 has been known as caused by a defect in sealing of the assembled portions of solid rocket propulsion motor.
In cases of solid rocket booster motors of space shuttles which has been presently developed by National Aeronautics and Space Administration (NASA), there is a problem about a damage of the assembled portions, since their assembling is carried out at atmosphere. Although several attempts for changing the design of the nozzle assembling parts were made, this problem could not be basically solved yet. It has been known that two of three kinds of O-rings disposed at the assembled portions were still subjected to a damage (Reference: AIAA 91-2292-CP, AIAA 89-2773) .